Acid-Cu plating is an old and successful technology. Although simple in principle, it relies in general practice on the use of proper additives to determine the properties of the Cu deposit. All additive systems consist of at least two chemicals, and usually more, at low concentrations in the plating solution. If the concentrations change, or if the additive components get out of balance, the quality of the plated Cu deteriorates. Monitoring and control of additives is thus very important, especially as the technological demands on the Cu become more stringent. However, no really good additive control techniques are known to exist in the art. Additive control in Cu plating is a major scientific and technological challenge.
One method proposed for additive monitoring is cyclic voltammetry ("CV"). The CV technique has been described by R. Haak, C. Ogden, and D. Tench, Plating 64(4):52 (April 1981) and 65(3):62 (March 1982). The basis of CV analysis is that the additives change the polarization of the Cu-deposition reaction and therefore affect the amount of Cu deposited in a linear potential sweep applied to an inert electrode in the plating solution. The measured copper plating charge is used as a measure of the additive concentration.
Cyclic voltammetry analyses are most commonly called cyclic voltammetry stripping ("CVS") and use the stripping or anodic charge as a measure of the amount of plated copper. The use of the stripping charge gives inferior results for analyses of Cu additives. Acid Cu plates with essentially 100% current efficiency as a two-electron reduction of Cu.sup.2+. In the presence of additives, however, Cu strips in a combination of +1 and +2 oxidation states. The stripping charge can therefore give an erroneous measure if it is assumed to be the same as the plating charge. The direct measurement of the plating charge presents no problem, however, with the proper instrumentation.
Despite claims that CV can be used as a monitoring tool (and the availability of a commercial CVS instrument), many serious questions about the technique still arise. The method does not measure a quantity that can be directly related to the concentration of components of a known solution. Additionally, one quantity, a charge, is used to estimate the solution level of a multi-component additive. For CV monitoring to be useful, the ratios of the components of the additive system must remain constant as the additive is consumed.
The present invention overcomes the deficiencies and problems associated with the conventional technology.